Prevention of corrosion and methods and techniques of preventing corrosion are of great interest in many different industries and across many different fields. One such field is military applications, where corrosion resistant materials are applicable to the protection of military vehicles such as tanks, transports, helicopters, and airplanes. Perhaps more importantly, corrosion resistance is crucial in naval vessels and submarines, which come in contact with seawater. It is known that corrosion resistance can be improved by the used of structurally designed materials in the amorphous state where the atoms are arranged in a non-periodic fashion. In general, corrosion properties are attributed to both the atomic level and the microstructure level. At the atomic level, periodic defects exist which may create pathways for attack by ionic oxygen, nitrogen and/or hydrogen, which can travel through the crystal without significant obstruction. Grain boundaries and voids exist in crystalline materials, which are avenues for chemical attack into materials, substantially lowering their corrosion resistance. Crystalline materials often have anisotropic thermal expansion properties Thermal cycling can change microstructures, resulting in additional grain boundaries, dislocations, fractures and voids, which can initiate stress corrosion cracking.
In amorphous metals, also called metallic glasses when prepared from the molten state, atomic arrangements are essentially random. Changes in the precise atomic locations do not significantly affect material properties. In these structures, thermal expansion can be highly isotropic, and grain boundaries and other defects can be eliminated. These structural changes mitigate stress corrosion cracking, and increase corrosion resistance even though local short range chemical order does occur in amorphous materials. Amorphous materials can be elementally tailored to specific applications. Since amorphous materials do not have a sharply defined melting point, they can be heat-softened and mechanically shaped. Metallic glasses often exhibit extraordinary mechanical and thermal properties, magnetic behavior, and corrosion resistance.
High-iron amorphous metal alloys containing minor amounts of other elements have been designed for corrosion resistant applications. The atomization process used to prepare large quantities of iron-based amorphous alloys is compositionally limited due to restraints on the cooling rate necessary to achieve an amorphous state. This is called the critical cooling rate (CCR). When the CCR is not achieved, some crystallization occurs. Only a particular compositional range can effectively yield amorphous solids using conventional fabrication techniques.
Iron-based amorphous alloys have been produced by various techniques, for example, by atomization, melt spinning, and casting. The material mixtures are first melted and then quickly quenched to room temperature. The required CCRs are normally 104 to 1011 Kelvin per second in order to achieve an amorphous structure. Atomized powders are thermal spray coated onto substrates using the high-velocity oxy-fuel (HVOF) process. Melt-spun ribbon samples of the same materials have also been prepared for testing purposes. Corrosion testing of iron-based amorphous ribbons suggests that corrosion resistance can be improved by increasing the alloy molybdenum content. However, it has heretofore been impossible to create an amorphous alloy with an appropriately high molybdenum content due to the high CCRs that are required.
Thus, current methods of amorphous alloy production are limited in what composition can be formed due to the process employed and the inherent requirement of high CCR. Therefore, it would be very beneficial to provide more flexibility in the composition of iron-based amorphous metal alloys by employing a more robust process of formation, resulting in more useful and previously unavailable coatings and/or structures with enhanced mechanical and/or thermal properties, magnetic behavior, and corrosion resistance.